Many types of vacuum arc coating apparatus utilize a cathodic arc source, in which an electric arc is formed between an anode and a cathode plate in a vacuum chamber. The arc generates a cathode spot on a target surface of the cathode, which evaporates the cathode material into the chamber. The cathodic evaporate disperses as a plasma within the chamber, and upon contact with the exposed surfaces of one or more substrates coats the substrates with the cathode material, which may be metal, ceramic, etc. An example of such an arc coating apparatus is described in U.S. Pat. No. 3,793,179 issued Feb. 19, 1974 to Sablev, which is incorporated herein by reference.
An undesirable result of the vacuum arc coating technique is the creation of macroparticles, which are formed from molten cathode material vaporized by the arc. These macroparticles are ejected from the surface of the cathode material, and can contaminate the coating as it is deposited on the substrate. The resulting coating may be pitted or irregular, which at best presents an aesthetic disadvantage, but is particularly problematic in the case of coatings on precision instruments.
In order to reduce the incidence of macroparticles contacting the substrate, conventionally a vacuum arc coating apparatus may be constructed with a filtering mechanism that uses electromagnetic fields which direct or deflect the plasma stream. Because macroparticles are neutral, they are not influenced by these electromagnetic fields. Such an apparatus can therefore provide a plasma duct between the cathode chamber and a coating chamber, wherein the substrate holder is installed off of the optical axis of the plasma source. Focusing and deflecting electromagnets around the apparatus thus direct the plasma stream towards the substrate, while the macroparticles, uninfluenced by the electromagnets, would continue to travel in a straight line from the cathode. An example of such an apparatus is described and illustrated in U.S. Pat. No. 5,435,900 issued Jul. 25, 1995 to Gorokhovsky for an “Apparatus for Application of Coatings in Vacuum”, which is incorporated herein by reference.
Another such apparatus is described in the article “Properties of Tetrahedral Amorphous Carbon Prepared by Vacuum Arc Deposition”, Diamond and Related Materials published in the United States by D. R. McKenzie in 1991 (pages 51 through 59). This apparatus consists of a plasma duct made as a quarter section of a tore surrounded by a magnetic system that directs the plasma stream. The plasma duct communicates with two chambers, one chamber which accommodates a plasma source and a coating chamber which accommodates a substrate holder.
The configuration of this apparatus limits the dimensions of the substrate to be coated to 200 mm, which significantly limits the range of its application. Furthermore, there is no provision in the tore-shaped plasma duct for changing the configuration of the magnetic field, other than the magnetic field intensity. Empirically, in such an apparatus the maximum value of the ionic current at the exit of the plasma duct cannot exceed 1 percent of the arc current. This is related to the turbulence of the plasma stream in the tore, which causes a drastic rise in the diffusion losses of ions on the tore walls.
The apparatus taught by Gorokhovsky in U.S. Pat. No. 5,435,900 incorporates a plasma duct surrounded by the deflecting magnetic system, a plasma source and a substrate holder mounted in the coating chamber off of the optical axis of the plasma source, where the plasma source and the substrate holder are surrounded by the focusing electromagnets. The plasma duct is designed in the form of a parallelepiped with the substrate holder and the plasma source mounted on adjacent planes. The magnetic system that forces the plasma stream towards the substrate consists of linear conductors arranged along the edges of the parallelepiped. The plasma duct has plates with diaphragm filters connected to the positive pole of the current source and mounted on one or more planes (not occupied by the plasma source) of the plasma duct. These plates serve as deflecting electrodes to establish an electric field in a transversal direction relative to the magnetic field lines, to guide plasma flow toward the substrate to be coated.
The advantages provided by U.S. Pat. No. 5,435,900 to Gorokhovsky include increasing the range of dimensions of articles (substrates) which can be coated, and providing the user with the option of changing the configuration of the magnetic field in order to increase ionic current at the exit of the plasma duct to 2 to 3 percent of the arc current.
A deflecting electrode is also described in U.S. Pat. No. 5,840,163 issued Nov. 24, 1998 to Welty, which is incorporated herein by reference. This patent teaches a rectangular vacuum arc plasma source and associated apparatus in which a deflecting electrode is mounted inside the plasma duct and either electrically floating or biased positively with respect to the anode. However, this device requires a sensor, which switches the polarity of the magnetic field when the arc spot on the rectangular source has reached the end of the cathode, in order to move the arc spot to the other side of the cathode. This results in an undesirable period where the magnetic field is zero; the arc is therefore not continuous, and is not controlled during this period. This ‘pseudo-random’ steering method cannot consistently produce reliable or reproducible coatings.
If the potential of the deflecting electrode (Vd) located opposite the plasma source is greater than the potential of the plasma source wall (Vw), an electric field occurs between them. The intensity of the electric field is given by:
                    E        ∝                                            V              d                        -                          V              w                                d                ∝                              σ            ⁡                          [                              1                +                                                      (                                                                  ω                        e                                            ⁢                                              τ                        e                                                              )                                    2                                            ]                                ⁢                      I            d                                              (        1        )            whered is the distance between the plate and the plasma duct wall,ωe is the gyro frequency of magnetized plasma electrons,τe is the characteristic time between electron collisions,σ is the specific resistivity of the plasma in the absence of a magnetic field, andId is the current of the deflecting electrode.
Because ωe is proportional to the plasma-guiding magnetic field B, (i.e. ωe∝B), the transversal electric field Et as determined by formula (1) will be proportional to B2, as shown by the following equation:Et∝σ└1+(ωeτe)2┘Id∝Bt2Id  (2)where Bt is the component of the magnetic field which is tangential to the surface of the deflecting electrode.
An ion is influenced by the force:Fi=Qi×Ei  (3)where Qi is the ion charge. Combining formulae (2) and (3) yields:Fi∝QiBt2Id  (4)This force causes the ion to turn away from the wall opposite the plasma source and directs it towards the substrate to be coated.
In the prior art, most of the surface of the deflecting electrode is disposed in a position where the transversal component of the magnetic field is strong and the tangential component of the magnetic field is relatively weak, which results in low magnetic insulation along the deflecting electrode. This is a disadvantage of the systems taught by Gorokhovsky and Welty, as it results in a weak deflecting electric field which is not strong enough to change the trajectory of heavy metal ions, such as Gf+ and W+, toward the substrate to be coated. Even in the case lighter ions such as Al+ and Ti+ the degree of ion deflection is slight, which results in substantial losses of metal ions before the plasma reaches the position of the substrate(s).
Another method used to reduce the incidence of macroparticles reaching the substrate is a mechanical filter consisting of a baffle, or set of baffles, interposed between the plasma source and the plasma duct and/or between the plasma duct and the substrate. Filters taught by the prior art consist of simple stationary baffles of fixed dimension, such as is described in U.S. Pat. No. 5,279,723 issued Jan. 18, 1994 to Falabella et al., which is incorporated herein by reference. Such filters create large plasma losses and a very low plasma yield, because the baffles destroy the geometry of the plasma duct.
Other mechanical filtering mechanisms, such as that taught by U.S. Pat. No. 5,435,900 to Gorokhovsky, trap macroparticles by altering the path of the plasma stream off of the optical axis of the plasma source toward the substrate, and trapping macroparticles in a baffle disposed generally along the optical axis of the cathode. However, this solution affects macroparticles only and does not allow for control of the plasma composition in the coating chamber, for example where it would be desirable to expose the substrate(s) to an ionized plasma without a metal component, as in plasma immersed processes such as ion implantation, ion cleaning, ion nitriding and the like. As such, prior art vacuum coating apparatus is suitable for use only in plasma vapor deposition (PVD) processes and a separate apparatus is required for plasma immersed processes.